The present invention relates to a direct reduction process for the reduction of iron ores in a rotary kiln provided with air injection pipes along its length and using countercurrent flow of gas and charge, and more particularly, to an improved method and means for introducing the air flow into the kiln.
Various methods have been suggested and used for carrying out the direct reduction process using high volatile coal as heating agent and reductant in a rotary kiln. For example, in some of these processes the coal is fed into the kiln through the discharge end by mechanical or pneumatic means, such as disclosed in U.S. Pat. No. 3,505,060 to Heitmann, and in some it is fed at the center of or along the kiln, such as disclosed in U.S. Pat. No. 3,206,299 to Senior et al. However, considerable disadvantages have arisen in blowing all of the high volatile coal into the kiln from the discharge end, and in feeding such coal at the center of the kiln. Because air is supplied to the kiln at a constant rate, unless altered by the intervention of the operator, and the composition of the chamber gas is subject to fluctuation, the reducing and combustion processes are not uniform, and the control of the process is adversely affected. When the reducing agent used has a high content of volatile matter or moisture, such as is the case with many low grade coals, the pressure in the rotary kiln is also subject to general and local fluctuations which further affect control of the process and which lead to a nonuniform discharge of solids from the kiln. As the distribution of the coal throughout the kiln is so highly critical, addition of all the coal from the discharge end makes the process difficult to control for simple mechanical and metallurgical reasons.
Further, although benefits are derived in regard to fixed carbon consumption when feeding all the fuel and reductant requirements in the form of a high volatile coal from the discharge end of the kiln, the control of the operation can be very difficult due to the large amount of fuel and reductant that has to be fed and the need to have highly precise distribution of the fuel if a high degree of reduction is to be achieved. It has been found in practice that it is not possible to maintain this fuel distribution, and consequently variable reduction results. It has also been found that the incorporation of high volatile coal into the kiln bed at the discharge end of the kiln results in impaired reduction capability in the kiln due to variations in the CO/CO.sub.2 ratio in the bed and in the chamber gas, and that this situation tends to limit the degree of metallization to a level below that required for commercial practicality.
On the other hand, feeding of a high volatile coal from the feed end of a countercurrent flow system leads to a loss of volatile material in the first section or preheat zone of the kiln. These volatiles are removed by the combustion gas flow and are thus lost to the process and increase the heat value of the kiln off gas, and only a portion of the gases from the low temperature distillation of the coal can be used for the process. The increased heat value of the off gas can further cause operating difficulties in the off-gas exhaust or processing system.
The disadvantages of these various approaches have been overcome by feeding a portion of the coal from the discharge end of the kiln sufficient to control the temperature profile throughout the kiln and feeding the remaining portions of the coal at the feed end while ensuring that the coal from the discharge end is distributed in the kiln in such a manner that substantially no coal lands in the reducing zone within the last 15% of the kiln length and is distributed to within the feed end zone of the kiln. The rotary kiln is fitted along its length with air injection devices which blow air countercurrent to the general flow of reducing gases within the kiln to produce mixture therebetween. A system of this type is disclosed in U.S. Pat. No. 3,890,138 to Hockin, particularly for use in reducing ilmenite. However, while this latter technique improves upon the other coal feeding methods in the direct reduction process, when it is used for reducing iron ore to sponge iron, certain problems still remain in the content of the exhaust or off gases requiring special attention in the off-gas processing or cleaning system.
It has generally been the practice in the direct reduction art to direct the air supply within the kiln along its length toward the feed end in keeping with the early teachings of Moklebust, for example, in U.S. Pat. No. 2,829,042 and subsequently in U.S. Pat. No. 3,170,786, as well as the teaching in the previously cited U.S. Pat. No. 3,206,299 to Senior et al. However, Meyer et al in U.S. Pat. No. 3,235,375 teaches the directing of the air flow preferably toward the discharge end or in either or both directions to achieve improved heat distribution and more effective combustion of carbon monoxide gas while obviating localized overheating of the charge mixture and formation of wall accretions.
The practice in the process of Hockin, U.S. Pat. No. 3,890,138, when reducing iron ore to sponge iron has been to direct the air toward the discharge end of the kiln countercurrent to the reducing gas flow to enhance mixing. Such is the arrangement shown in the prior art diagram of FIG. 1 which illustrates the components of a direct reduction plant for producing sponge iron in accordance with the HOCKIN process. Although the HOCKIN process overcomes many of the problems of the prior art, still as noted above, difficulties have been encountered in handling the off-gases. The present invention involves certain improvements which have been discovered in the operation of the illustrated plant for the HOCKIN process.